The Federal Communications Commission (FCC) has issued a stringent set of regulations and specifications that must be satisfied by equipment intended for connection to a telephone line to prevent such equipment from degrading the operation of the network. This equipment, such as a modem, must satisfy these regulations and specifications before the equipment is connected to the telephone network. Generally, some form of electrical isolation circuitry is required to prevent voltage transients or surges generated by the equipment from reaching the telephone line and thereby degrading telephone network operation. Further, the connected equipment must not provide a path from the telephone line to ground.
The use of a transformer within a modem to electrically isolate the telephone line is well known. Typically, a directly coupled or capacitively coupled isolation transformer is connected between the two wire telephone line and the analog front end of the modem circuit to provide the necessary electrical isolation.
Although a transformer supplies a high degree of electrical isolation, the transformer is also one of the physically largest and heaviest electrical components in a typical modem circuit. The placement of the transformer on a modem printed circuit board having limited board layout space is a challenging design task because the transformer is much larger than the other electrical components of the remaining modem circuitry. Furthermore, the overall weight of a modem is typically significantly increased by the use of the isolation transformer in a modem design. Consequently, the physical size and weight of the modem, as well as the appearance of the packaging or housing for a modem, is often constrained by the physical characteristics of the isolation transformer.
A modem is often utilized with a portable computer system for communicating data between a remote site and a central computer location. Portable personal computers, such as laptop and notebook personal computers, are of small size and low weight and generally require peripheral devices of proportional small size and low weight. The portability of a laptop or a notebook computer is limited if a necessary peripheral device, such as a modem, is of significant size or weight when compared with the computer. Therefore, the users of such small size portable computers require a reduced sized modem package that is thinner and lighter than modems intended for use with larger personal computers employed in a stationary location, such as desktop personal computers.
The Personal Computer Memory Card International Association (PCMCIA) has established an industry standard for connecting a peripheral device to a small sized personal computer, such as a laptop or a notebook computer. To date, the small sized modems compatible with this industry standard, often described as thin modems, utilize an external "plug-in" or outboard isolation transformer to supply the proper voltage levels to the modem and to electrically isolate the telephone line from the computer. The transformer, which connects to a power outlet during modem operation, is physically separated from the remaining modem circuitry by a cable, thereby enabling the thin modem to be conveniently installed within or adjacent to the housing of the small sized computer. However, the portability of a small sized computer connected to a thin modem having an external isolation transformer remains limited by the bulky size and weight of the outboard transformer.
To reduce the size and weight of the isolation transformer, it is well known to provide one signal path for coupling certain telephone line signals, such as incoming ringing signals, to the modem circuitry and another signal path which couples the data signals to the modem circuitry via a capacitively coupled isolation transformer. This capacitive coupling design enables the use of a smaller and lighter isolation transformer by preventing saturation of the small core of the transformer. Nevertheless, a transformer is still required by this prior art design to provide the necessary isolation between the telephone line and the modem circuitry.
Alternative prior art modems utilize electro-optical isolators to isolate the telephone line from the data device. Electro-optical isolators typically employ an emitter, such as a light emitting diode (LED), which supplies a light signal in response to an input electrical signal, and a photodetector that supplies a corresponding electrical output signal in response to the the received light signal. The use of electro-optical isolators for electrically isolating an input circuit from an output circuit is well known.
U.S. Pat. Nos. 4,373,117, 4,417,099, and 4,578,533, all to Pierce, and U.S. Pat. No. 4,395,590 to Pierce et al. describe a line powered modem that derives its operating power entirely from the telephone line and uses electro-optical isolators within receiver and transmitter data buffers located between the telephone line and an RS-232 serial interface to electrically isolate the telephone line. However, the Pierce line powered modem also includes a DC-to-DC converter that utilizes a two-winding transformer to supply the proper voltage levels for the RS-232 interface and to provide additional electrical isolation between the telephone line.
U.S. Pat. No. 4,592,069 to Redding describes a line powered modem that obtains its entire operating power from the telephone line and uses an electro-optical isolator located at each of the data input terminals and the data output terminals of the modem. The electro-optical isolators electrically isolate the terminals from the remaining modem circuitry to provide the required isolation for the telephone line. The Redding line powered modem further uses an inductance simulation circuit to prevent the power supply from loading down the AC signal on the telephone line, thereby assisting the construction of the modem circuit as a single integrated circuit. However, the power consumption of the Redding line powered modem is limited because the modem does not have a power source separate from the telephone line and FCC regulations limit the amount of power that can be drawn from the telephone line. Also, the Redding modem is a relatively simple modem having only a frequency shift keying (FSK) mode and does not support other high level or advanced communications modes, such as phase shift keying (PSK) or PSK combined with pulse amplitude modulation (PAM).
Therefore, there is a need for a small sized, advanced modem that has sufficient power capability to support high level communications modes and does not use a transformer for electrically isolating the telephone line from the data device. The present invention uses electro-optical isolators for coupling signals between a first modem circuit including a transmitter, a receiver and a controller, and a second modem circuit connected to the telephone line, thereby electrically isolating the telephone line. Electro-optical isolator devices, which are typically packaged within a dual in-line package, are sufficiently small to allow the construction of a thin modem. The present invention further uses separate power sources to power the first and second modem circuits, thereby supplying the necessary power to support advanced communications modes, and to maintain the electrical isolation provided by the electro-optical isolator devices connected between these modem circuits.